1. Field of the Invention
The present invention relates to a unitized replaceable drilling common bit, and more particularly to a cutting tip of the drilling common bit, in which the central drill has an extended cutting edge, thereby to form a continuous cutting with the side cutting edges so as to prevent the cutting edge from chipping by built-up edge, and make the central drill become smoother and tougher, further to improve cutting property, easy manufacturing and efficiently reducing production cost, and improving competition capability in the market.
2. Description of Prior Art
In accordance with the conventional composite drilling common bit 10, manufactured by the famous provider KENNAMETEL HERTEL Co., as shown on FIG. 8, beside the chisel-edge (static point) 12 of the cutting tip 11 the both cutting edges have several chip-breaker grooves 13 for breaking the chip into smaller pieces, but these chip-breaker grooves 13 make the cutting lips have discontinuous cutting so that the chip may pile up in there, and due to the slow spinning at the chisel-edge 12, the bit may bear relative greater cutting resistance and pressure, therefore the chip may be held up there and the corners of the chip-breaker grooves 13 bear force so big that they are broken up to occur the cutting tip 11 damage so as to unable to be used any more, so this shortcoming will increase the tool cost.
And referring to FIG. 9, an other kind of unitized drilling common bit 20, provided by KENNAMETEL HERTEL Co. too, includes a central drill 21 embedded at the center, an inside cutting insert 22 and an outside cutting insert 23 secured on the bit shank, but in this structure, the gaps between the central drill 21's chisel-edge and the side cutting edges, and the adjacent side cutting inserts—inside and outside cutting inserts 22 and 23 are easy to hole up chip (the portions fingered by arrows a and b) so as to extrude the cutting edges, and additionally spinning in lower speed, the relative cutting resistance is gone up, the cutting edges are easy to be broken, especially to the portion fingered by arrow 1 between the side cutting edge of the central drill 21 and the adjacent inner cutting corner of the inside cutting insert 22, the cutting stress concentrates on this corner so as to break it easily to damage the cutting insert, because the central drill 21 is expensive, so the relative production cost may go up.